The demand for electro-chemical power cells, such as Lithium-ion batteries, is ever increasing due to the growth of applications such as electric vehicles and grid storage systems, as well as other multi-cell battery applications, such as electric bikes, uninterrupted power battery systems, and lead acid replacement batteries. It is a requirement for these applications that the energy and power densities are high, but just as important, if not more, are the requirements of low cost manufacturing and increased safety to enable broad commercial adoption. There is further a need to tailor the energy to power ratios of these batteries to that of the application.
For grid storage and electric vehicles, which are large format applications multiple cells connected in series and parallel arrays are required. Suppliers of cells are focused either on large cells, herein defined as more than 10 Ah (Ampere hours) for each single cell, or small cells, herein defined as less than 10 Ah. Large cells, such as prismatic or polymer cells, which contain stacked or laminated electrodes, are made by LG Chemical, AESC, ATL and other vendors. Small cells, such as 18650 or 26650 cylindrical cells, or prismatic cells such as 183765 or 103450 cells and other similar sizes are made by Sanyo, Panasonic, EoneMoli, Boston-Power, Johnson Controls, Saft, BYD, Gold Peak, and others. These small cells often utilize a jelly roll structure of oblong or cylindrical shape. Some small cells are polymer cells with stacked electrodes, similar to large cells, but of less capacity.
Existing small and large cell batteries have some significant drawbacks. With regard to small cells, such as 18650 cells, they have the disadvantage of typically being constrained by a an enclosure or a ‘can’, which causes limitations for cycle life and calendar life, due in part to mechanical stress or electrolyte starvation. As lithium ion batteries are charged, the electrodes expand. Because of the can, the jelly roll structures of the electrodes are constrained and mechanical stress occurs in the jelly roll structure, which limits its life cycle. As more and more storage capacity is desired, more active anode and cathode materials are being inserted into a can of a given volume which results in further mechanical stresses on the electrode.
Also the ability to increase the amount of electrolyte in small cells is limited and as the lithium intercalates and de-intercalates, the electrode movement squeezes out the electrolyte from the jelly roll. This causes the electrode to become electrolyte starved, resulting in concentration gradients of lithium ions during power drain, as well as dry-out of the electrodes, causing side reactions and dry regions that block the ion path degrading battery life. To overcome these issues, especially for long life batteries, users have to compromise performance by lowering the state of charge, limiting the available capacity of the cells, or lowering the charge rate.
On the mechanical side, small cells are difficult and costly to assemble into large arrays. Complex welding patterns have to be created to minimize the potential for weld failures. Weld failures result in lowered capacity and potential heating at failed weld connections. The more cells in the array the higher the failure risk and the lower manufacturing yields. This translates into higher product and warranty costs. There are also potential safety issues associated not only by failure issues in welds and internal shorts, but also in packaging of small cells. Proper packaging of small cells is required to avoid cascading thermal runaway as a result of a failure of one cell. Such packaging results in increased costs.
For large cells, the disadvantages are primarily around safety, low volumetric and gravimetric capacity, and costly manufacturing methods. Large cells having large area electrodes suffer from low manufacturing yields compared to smaller cells. If there is a defect on a large cell electrode more material is wasted and overall yields are low compared to the manufacturing of a small cell. Take for instance a 50 Ah cell compared to a 5 Ah cell. A defect in the 50 Ah cell results in 10× material loss compared to the 5 Ah cell, even if a defect for both methods of production only occurs every 50 Ah of produced cells
Another issue for large cells is safety. The energy released in a cell going into thermal runaway is proportional to the amount of electrolyte that resides inside the cell and accessible during a thermal runaway scenario. The larger the cell, the more free space is available for the electrolyte in order to fully saturate the electrode structure. Since the amount of electrolyte per Wh for a large cell typically is greater than a small cell, the large cell battery in general is a more potent system during thermal runaway and therefore less safe. Naturally any thermal runaway will depend on the specific scenario but, in general, the more fuel (electrolyte) the more intense the fire in the case of a catastrophic event. In addition, once a large cell is in thermal runaway mode, the heat produced by the cell can induce a thermal runaway reaction in adjacent cells causing a cascading effect igniting the entire pack with massive destruction to the pack and surrounding equipment and unsafe conditions for users.
When comparing performance parameters of small and large cells relative to each other, it can be found that small cells in general have higher gravimetric (Wh/kg) and volumetric (Wh/L) capacity compared to large cells. It is easier to group multiples of small cells using binning techniques for capacity and impedance and thereby matching the entire distribution of a production run in a more efficient way, compared to large cells. This results in higher manufacturing yields during battery pack mass production, in addition, it is easier to arrange small cells in volumetrically efficient arrays that limit cascading runaway reactions of a battery pack, ignited by for instance an internal short in one cell (one of the most common issue in the field for safety issues). Further, there is a cost advantage of using small cells as production methods are well established at high yield by the industry and failure rates are low. Machinery is readily available and cost has been driven out of the manufacturing system.
On the other hand, the advantage of large cells is the ease of assembly for battery pack OEMs, which can experience a more robust large format structure which often has room for common electromechanical connectors that are easier to use and the apparent fewer cells that enables effective pack manufacturing without having to address the multiple issues and know-how that is required to assemble an array of small cells.
In order to take advantage of the benefits of using small cells to create batteries of a larger size and higher power/energy capability, but with better safety and lower manufacturing costs, as compared to large cells, assemblies of small cells in a multi-core (MC) cell structure have been developed.
One such MC cell structure, developed by BYD Company Ltd., uses an array of MC's integrated into one container made of metal (Aluminum, copper alloy or nickel chromium). This array is described in the following documents: EP 1952475 AO; WO2007/053990; US2009/0142658 A1; CN 1964126A. The BYD structure has only metallic material surrounding the MCs and therefore has the disadvantage during mechanical impact of having sharp objects penetrate into a core and cause a localized short. Since all the cores are in a common container (not in individual cans) where electrolyte is shared among cores, propagation of any individual failure, from manufacturing defects or external abuse, to the other cores and destruction of the MC structure is likely. Such a cell is unsafe.
Methods for preventing thermal runaway in assemblies of multiple electrochemical cells have been described in US2012/0003508 A1. In the MC structure described in this patent application, individual cells are connected in parallel or series, each cell having a jelly roll structure contained within its own can. These individual cells are then inserted into a container which is filled with rigid foam, including fire retardant additives. These safety measures are costly to produce and limit energy density, partly due to the excessive costs of the mitigating materials.
Another MC structure is described in patent applications US2010/0190081 A1 and WO2007/145441 A1, which discloses the use of two or more stacked-type secondary batteries with a plurality of cells that provide two or more voltages by a single battery. In this arrangement single cells are connected in series within an enclosure and use of a separator. The serial elements only create a cell of higher voltage, but do not solve any safety or cost issues compared to a regularly stacked-type single voltage cell.
These MC type batteries provide certain advantages over large cell batteries; however, they still have certain shortcomings in safety and cost.